Occasionally, if you're troubleshooting your PC or installing a new operating system, you'll have to "boot it from a disc" or USB flash drive. Here's what that means, and how to do it.

When you press the power button on your computer, it boots up from its internal hard drive, where your operating system (usually Windows, OS X, or Linux) is stored. But sometimes, you need to boot into something different. Maybe you need to boot from your Windows installation disc to reinstall Windows, or perhaps you need to boot from a system rescue disc to solve a problem with your machine. When you're told to boot from a CD or USB drive, here's what you need to do.

On a Windows PC

Every Windows PC is different, but once you get into your system's boot menu you should be able to find what you're looking for. To boot from a CD or USB drive:

Restart your computer and wait for that first screen to pop up. Often, it'll say something like "Press F12 to Choose Boot Device" somewhere on the screen. Press that key now.

Give it a moment to continue booting, and you should see a menu pop up with a list of choices on it. Highlight your CD or USB drive and press Enter.

Alternatively, you can set your computer to always check for a bootable CD or USB drive. That way, when you have one inserted, it'll boot from it automatically, and when you don't, it'll head into your regular operating system. To set this up, follow these instructions:

Restart your computer and watch for the first screen that shows up when you boot. You should see something that says "Press DEL to enter setup," or something similar. Press the noted key on your keyboard and wait for setup to start.

You should see a new screen pop up, called your BIOS. This is where you set a lot of low-level settings for your computer. Look through the options (being careful not to change anything) for a setting called "Boot Device," "Boot Order," or something similar. Select that option.

From the menu that pops up, choose your computer's disc drive and press Enter. If you're trying to boot from a USB drive, choose USB-HDD instead and press Enter. Depending on your BIOS, you may need to use the Page Up and Page Down keys to move your selection to the top of a list, instead.

Exit out of your BIOS, saving your changes. Usually, this option is under "Exit" on the main menu, or available via a keyboard shortcut listed somewhere on your screen.

Your computer should reboot. Make sure your CD or USB drive is in your computer. If you're prompted to "Press any key to boot from CD/DVD," do so. Your computer should boot into the CD or USB drive instead of your normal operating system.

From there, you can follow the instructions on-screen to install Windows, troubleshoot issues, or do whatever else it is you need to do. Image by TechnoGeek95.

On a Mac

Booting from CD or USB is very, very easy on a Mac. All you need to do is:

Reboot your computer. When the white screen first shows up and you hear the startup chime, press and hold the "Option" key.

You should see a list of drives show up. On the right, you'll find your CD or USB drive, listed with its name under it. Use your arrow keys or your mouse to highlight that drive, then press Enter or click on the arrow below it.

From there, your computer will boot into the CD or USB drive instead of OS X, and you can do whatever it is you need to do following the instructions on-screen.

It's a pretty simple process once you get used to it, and it can be immensely useful if you're taking the maintenance of your computer into your own hands.

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